1. Field of the Invention
The present invention relates generally to communication systems, and more specifically to source termination in communication systems.
2. Background
Devices in a communication system generally include transmitters to transmit information via an electrically conductive medium, such as a transmission line. For instance, a transmitter in a first device can transmit information to a receiver in a second device, and a transmitter in the second device can transmit information to a receiver in the first device. The transmit and receive functions of a device are often combined using a transceiver.
Components of the communication system that are coupled to a transmitter are cumulatively referred to as the load of the transmitter. The load has a load impedance, and the transmitter has a source impedance. The load impedance, and the source impedance are often matched to facilitate the transfer of power from the transmitter to the load.
A source termination can facilitate matching the source impedance and the load impedance and/or absorb reflections on a transmission line to which the source termination is connected. However, conventional source terminations are off-chip in order to achieve precision output voltage amplitudes, greater linearity, and/or higher bandwidth. For example, the source termination is usually a discrete precision resistor coupled to an integrated circuit (IC) chip that includes the transmitter. Parasitics between the chip and the off-chip source termination can cause hybrid residual for bi-directional communication systems. Off-chip source terminations can cause voltage peaking or voltage overshoot inside the transmitter and/or at the load. The return loss performance of off-chip source terminations degrades substantially at higher frequencies. For instance, the return loss associated with an off-chip source termination can be less than 5 dB at frequencies greater than approximately 400 MHz.
What is needed, then, is a source termination that addresses one or more of the aforementioned shortcomings of conventional source terminations.